Microsoft Windows SAM Local Privilege Escalation Vulnerability
CVE-2021-36934 is a security vulnerability in Microsoft Windows, added to CISA KEV on February 10, 2022. Patch priority: Plan to patch.
What it is
If a Volume Shadow Copy (VSS) shadow copy of the system drive is available, users can read the SAM file which would allow any user to escalate privileges to SYSTEM level.
VulnCheck tracked exploitation 1 day before CISA's KEV listing. VulnCheck's broader catalog of in-the-wild exploitation evidence picked this up before it landed on CISA's formal list. Per-CVE fact about this entry; not a population claim about lead time in general.
Who's affected
Affects anyone running Windows workstations or servers. In a small CPA, legal, or dental practice, Windows is typically the platform your accounting, document management, or practice management software runs on — exploitation gives an attacker access to whatever client files and credentials live on those machines.
What to do
Apply updates per vendor instructions.
If you don't have someone in-house to verify the patch deployed across every endpoint — or you're not sure whether you're affected — that's exactly the kind of triage we do. Book a free 20-minute triage call.
Insurance readiness
Your insurer will ask if this is patched.
Unpatched entries on CISA's Known Exploited Vulnerabilities list are exactly what cyber-insurance carrier questionnaires probe for, and knowing which controls they check is how you keep a renewal from stalling.
Get the free carrier questionnaire →Severity
CVSS base score: 7.8 — HIGH
Microsoft fix
Microsoft Exploitability Index (assessed at update release): Exploitation More Likely
This CVE is on CISA's Known Exploited Vulnerabilities catalog. Microsoft's index reflects their release-time forecast, not current observed activity, and is not automatically revised upward when exploitation appears later.
Vendor workaround
Vendor-published workaround
We recommend installing this security update as soon as possible. If you must delay installation of this security update, we recommend this workaround:
Restrict access to the contents of %windir%\system32\config
Command Prompt (Run as administrator): icacls %windir%\system32\config\*.* /inheritance:e
Windows PowerShell (Run as administrator): icacls $env:windir\system32\config\*.* /inheritance:e
Delete Volume Shadow Copy Service (VSS) shadow copies
Delete any System Restore points and Shadow volumes that existed prior to restricting access to %windir%\system32\config.
Create a new System Restore point (if desired).
Impact of workaround Deleting shadow copies could impact restore operations, including the ability to restore data with third-party backup applications. For more information on how to delete shadow copies, see KB5005357- Delete Volume Shadow Copies.
Note 1 You must restrict access and delete shadow copies to mitigate this vulnerability.
Note 2 Even after installing this security update, you must delete all shadow copies of your system volume to fully mitigate this vulnerability.
Caution Restoring your system from a backup could also restore the overly permissive ACLs, and therefore revert your system to a vulnerable state. After restoring a backup, you must verify that the ACLs are correct to ensure that the restore operation did not reintroduce this vulnerability.
A workaround reduces exposure while you patch — it is not a substitute for the update. Install the fix above as soon as you can.
CISA due date
Federal deadline: February 24, 2022. Federal agencies must complete the required action by this date. For private SMBs the deadline is advisory — but treat it as a strong recommendation, especially if you handle regulated data (HIPAA, GLBA, ABA model rules).
EPSS
EPSS score: 67.3% — higher than 99% of all scored CVEs.
EPSS estimates the probability of exploitation activity in the next 30 days. Every entry on this site is already confirmed exploited, so read it as relative urgency among the things you still have open, not as permission to wait.
Other exploited Microsoft Windows entries
More CISA KEV entries for Microsoft Windows. Same product line, same actively-exploited status.
Source
This entry comes from CISA's Known Exploited Vulnerabilities catalog, public US-Government data we pull daily. The plain-language framing and the what-to-do guidance above are ours. View the original entry on cisa.gov.